StereoGraphics Corporation of San Rafael, Calif., introduced the push-pull ZScreen® modulator using pi-cell technology for direct viewing of stereoscopic images on monitors in 1987. The technology used in these products is described in U.S. Pat. No. 4,792,850 to Lipton et al. entitled Method and System Employing a Push-Pull Liquid Crystal Modulator. A version of the push-pull product continues to be manufactured by StereoGraphics for use in conjunction with high-end CRT based projectors.
In 1998, StereoGraphics re-introduced a pi-cell modulator having a different design, which is intended for use with a CRT based monitor image. The technology basis for this product is described in an article by L. Lipton, et. al., entitled “An Improved Byatt Modulator,” Stereoscopic Displays and Virtual Reality Systems V, Vol. 3295, pp. 121-126, SPIE, 1998, and in disclosed in co-pending application Ser. No. 09/381,916, which has been published as WO 98/44746.
There is a considerable body of literature that describes the functioning of the pi-cell, including the following: U.S. Pat. No. 4,884,876 to Lipton et al., U.S. Pat. No. 4,719,507 to Bos, and U.S. Pat. No. 4,566,758 to Bos.
The following references discuss the use of a pi-cell device in the form of a large modulator for field switching: High-Performance 3-D Viewing Systems Using Passive Glasses by Bos et al. (p. 450, SID '88 Digest) and Field-Sequential Stereoscopic Viewing Systems Using Passive Glasses by Haven (Proceedings of the SID, vol. 30/1, 1989). In addition, Johnson and Bos, in their article Stereoscopic Display Performance (ELECTRONIC IMAGING EAST CONFERENCE, Building Application Solutions with Today's Imaging Tools, 1990), describe in detail how the Byatt shutter improves performance in terms of suppression of ghosting created by phosphor afterglow.
Twisted-nematic (TN) technology, the most commonly used liquid crystal technology, owes its electro-optical effect to optical activity, which is produced by light traversing the bulk of the material. The physics of pi-cells, on the other hand, is explained by a phase shift created by retardation occurring at the surface layer or that immediately adjacent to the director alignment layer. It is this phase shift that enables the pi-cell to modulate or even produce circularly polarized light that makes it particularly interesting for stereoscopic display applications. At one time it was the speed of the pi-cell that was considered to be its most important attribute, but its speed has been matched in recent years by improved twisted-nematic devices.
The most important characteristic of the pi-cell is that it is a phase shifting device, and thus it can readily produce circularly polarized light.
Because pi-cell properties are so heavily dependent upon the surface effect, the device tends to have difficulties associated with this boundary region and is especially sensitive to rub defects and ion contamination. In addition, while TN parts usually become fully functional, i.e., turn on rapidly, pi-cells may take many seconds to go fully pi. The result can be disturbing artifacts which appear as hazy, mottled, or streaked areas, which greatly detract from the pleasure of the viewing experience.
The difficulties can be broken down into two types: those which occur upon start-up and which may well eventually clear with the passage of time, and those that persist indefinitely. Accordingly, we have developed means for overcoming both kinds of defects.
The traditional technique for driving pi-cells has been to use a waveform modulated by a carrier with a frequency of one to two kHz. However, we have found that for pi-cell parts made in some factories, the technique is not a good one. We have therefore created a unique driving approach using a modified carrier waveform, and in addition, what we term a “stutter start,” to overcome the artifacts described above.
As mentioned above, StereoGraphics has been producing products utilizing pi-cell technology for viewing of stereoscopic images since 1987. The original product was a ZScreen® modulator that was placed in front of a monitor. This particular configuration was effectively phased out with the introduction of CrystalEyes® modulating eyewear in 1989. StereoGraphics now manufactures a different configuration of the ZScreen product for use in conjunction with high-end CRT based projectors. The current ZScreen product uses a modest size (6 inches square) liquid crystal panel.
As noted above, in 1998, we reintroduced a pi-cell modulator of a different design, which is intended for use with a CRT based monitor image. This is a large panel (16″ by 12¼″) and as such, the material cost is relatively expensive. We were looking for ways to reduce our costs by improving the yield.
Our yields were being affected by a number of factors, many of which we were able to address in the manufacturing process. Despite our process improvements, we continued to produce a number of liquid crystal cells that failed our previous quality standards. Rather than lowering our standards to accept these cells, we sought a technique to drive them with a different waveform. By this means we hoped the cells would pass our inspections.
We were concerned with two defects that occurred frequently. One problem that we encountered was the reluctance of a particular cell (or more specifically a small portion of a cell) to “go pi.” By that we mean that parts of the cell did not properly modulate the polarization in response to the driving waveform. While the majority of the cell area performed properly, it was common for some areas to take a few seconds or even a few minutes before reaching complete effectiveness. After the cell went pi, the modulating effect was completely normal. This problem would recur if the cell had been inactive for as little as a few seconds.
The other problem that we encountered was a visible “shadow” in an area of a cell that developed during normal operation. It would not normally be visible until after many minutes (or even hours) of operation. Invariably this shadow defect took the form of a small triangle 101 located at the lower right of each of the five electrode segments of the exemplary cell 102 as shown in FIG. 1. We believe the cause of this defect to be free ions contaminating the liquid crystal material, and as such we call this defect the “ion shadow” defect.
The number of cells that had either defect was low. The vast majority of cells went pi in less than five seconds, and did not develop ion shadows even after hundreds of hours of operation.
Since the inception of pi-cells, they have generally been driven by an alternating polarity waveform of the sort shown in FIG. 2. Bursts of a carrier 201 of 1-2 kHz or so, which activate the cell, occur every other field. When the cell is inactive, the voltage across it is zero. This waveform has a net DC value of zero volts, with the result that the integral of the voltage applied across the cell over a long period of time is zero. The cell spends the same amount of time with a positive voltage across it as it does with a negative voltage across it. This is required to prevent the breakdown of the cell through transmigration of the electroplating from one electrode to the other.
StereoGraphics developed an alternate driving waveform, shown in FIG. 3, which is used in a number of products. This “quasi-static” waveform retains the net DC value of zero volts but eliminates the carrier by inverting every other field. A positive drive signal 301 is applied for a time equal to one field. During the next field the drive signal 302 is zero. A negative drive signal 303, exactly equal in amplitude and opposite in polarity is applied during the third field. The fourth field drive signal 304 is zero once again. The four-field pattern repeats indefinitely.
A variation of this waveform, shown in FIG. 4, is used in the current Monitor ZScreen product. In the modified quasi-static waveform, a small bias voltage is placed across the cell when it is not activated. This bias voltage allows the segments of the cell to appear more uniform, thus making the segment boundaries less noticeable. The positive and negative drive signals 401 and 403 are equal and analogous to the drive signals 301 and 303 in FIG. 3. The difference lies in the off-state drive signals. In these off-states, a small bias voltage is applied, first a negative bias voltage 402 (opposite polarity to 401), then after 403, a positive 404 bias voltage (opposite polarity to the previous drive voltage).